Method for monitoring seafloor subsidence and for gravity monitoring an underground hydrocarbon reservoir

ABSTRACT

The invention is a method for monitoring subsidence of the sea-bed ( 14 ) of a survey area ( 8 ) caused by compaction of an underground hydrocarbon reservoir ( 1 ), and comprises the following steps: Conducting at least two series (S 1 , . . . ,S i , . . . ,S m ) of time-indexed depth measurements ( 13   a   , . . . ,13   n ), with separation in time Δ between the measurement series characteristic of a significantly detectable long-term change of seafloor elevation due to compaction to take place in the reservoir. Measurements are time-indexed and corrected for tidal depth variations. Depth measurements ( 13 ) are conducted on survey stations ( 2 ) arranged on benchmarks ( 6 ) which have settled in the locally consolidated seabed ( 14 ). To handle short-term depth variations several stationary time-indexed short-time local reference depth measurement series ( 19   r ) are conducted at short-term local reference stations ( 18   r ) at benchmarks ( 6 ) during each separate measurement series (S i ). The reference depth measurement series ( 19   r ) are continuous for correcting each depth measurement ( 13 ) for short-time tidal depth variations occuring during each separate measurement series (S i ). To monitor compaction or seafloor subsidence during the series of measurements, the depth measurements ( 13, 19   r ) are conducted relative to a depth measurement ( 13   r ) during each series (S) at reference station ( 9 ) arranged on the seabed ( 14   r ) at a distance from the reservoir ( 8 ) being sufficiently far to be unaffected by long-term effects taking place due to compaction in reservoir ( 1 ).

TECHNICAL FIELD OF THE INVENTION

The invention concerns a method and an apparatus for monitoring fluidmovements and seafloor subsidence/reservoir compaction inhydrocarbon-producing fields by repeated relative seafloor gravity anddepth measurements. Monitoring of hydrocarbon reservoir changes (assaturation, pressure, and compaction) during production is traditionallydone by well measurements integrated through dynamic reservoirmodelling. Geophysical techniques for measuring changes between thewells has emerged as useful technology in recent years, particularlyrepeated seismic measurements. The benefits of such observations andimproved understanding of the reservoir behaviour during production aremany: among others to optimize production/reservoir management,optimizing drilling of infill wells and improving estimates of remainingreserves.

TECHNICAL BACKGROUND, STATE OF THE ART

A new system comprising an instrument for use in seafloor gravityobservations has been designed and deployed. The system is called“Remotely Operated Vehicle Deep Ocean Gravimeter” (ROVDOG). The aim ofthe project was to perform repeated measurements of gravity and pressurein an oilfield to monitor the development of the reservoir. The actualfield in question is the Troll field in the North Sea. Because of therequirement for accurate location of the measurement points (each towithin one cm of the previous observation) a gravimeter was requiredwhich could be handled by the arm of an ROV and placed atop sea floorbenchmarks. Such an instrument has been designed around a Scintrex CG-3Mland gravimeter. Motorized gimbals within a watertight pressure case areused to level the sensor. An assembly of 3 precise quartz pressuregauges 22 provides pressure measurements which can be transformed todepth information. The instrument may be operator-controlled via aserial datalink to the ROV. A view of the data stream for recording canbe monitored. In one embodiment of the invention, the serial datalink isaccording to the RS-232 standard. In a test run of the system, theinstrument was first deployed in the Troll field during June 1998. Atotal of 75 observations were made at 32 seafloor locations over aperiod of 120 hours. The repeatability figure of merit is 0.027 milliGalfor the gravity measurement and 2 cm for pressure-derived heights.

Scope of Work

The Troll field licence partners decided in 1996 to try to performmeasurements on the Troll field in order to montitor changes caused bythe gas production and the influx of water from the aquifier inparticular. Among different solutions comprising well monitoring andrepeat seismic monitoring to perform such measurements, repeated gravitymeasurements on the sea floor above the field was proposed by theinventors. In an internal study in 1997 the changes of the gravity fieldwere identified as comprising the following factors:

I) water influx in the gas reservoir;

II) seafloor subsidence; and

III) gas density reduction.

I) The expected increase in gravity caused by water influx.

II) The expected seafloor subsidence due to reservoir compaction. Thisseafloor subsidence will cause a change in the gravity field, asmeasured at the seafloor, being proportional to the subsidence, due tothe vertical gradient of the Earth's gravity field. Thus it is necessaryto monitor the gravity changes or “the gravity signal” to within aresolution of the gravity corresponding to an elevation difference of afew centimeters.

III) Gas density reduction will give a reduction of the mass density ofthe reservoir, causing a reduction of the gravity field, i.e. ofopposite sign with respect to gas/water rise.

Fujimoto et al. in “Development of instruments for seafloor geodesy”,Earth Planets Space, vol. 50, pp. 905-911, 1998, describes instrumentsfor monitoring differential displacements across a fault zone in theseabed, and examines their resolutions through seafloor experiments atrelatively short baselines. The horisontal differential displacement ismeasured by an acoustic ranging system using a linear pulse compressiontechnique being able to measure distances on the order of 1 km betweenmarkers with an accuracy of 1 cm. The leveling or vertical displacementmonitoring of the seabed is planned to use an array of ocean bottompressure gauges and an ocean bottom gravimeter to detect differentialvertical motion. The system is estimated to have a resolution of severalcentimeters in vertical displacement. Fujimoto et al describes how oceanbottom pressure measurements can be used in two ways to detect verticalmovements of the seafloor. An ocean bottom pressure array acts as amonitoring system of relative vertical movements. Variations ofatmospheric pressure are mostly compensated at the sea surface. Bysimulating pressure and gravity one can discriminate between a pressurechange due to vertical seafloor displacements and a pressure change dueto vertical sea surface displacements:

Consider the seafloor rising by 1 cm. The pressure value will decreaseby 1 cm of water column. Gravity will decrease by 2.2 microGal (−3.068microGal due to height change and +0.864 microGal due to reducedgravitational attraction of the global sea water).

Next, Consider the sea surface lowering 1 cm. The pressure in this casewill also decrease by 1 cm of water column. The gravity in this casewill increase by 0.432 microGal due to the reduced gravitationalattraction of the local seawater.

In both of the above mentioned cases, pressure monitored at the seabeddecreases, but the gravity changes differently. If measurements areperformed with high accuracy, simultaneous measurements of pressure andgravity can discriminate between the two effects: sea surface levelchange and seabed level change.

Fujimoto et al. do not propose any method for monitoring changingparameters representing density and/or mass distribution of anunderground sub-sea reservoir by means of gravimetric measurements witha gravity sensor on the sea-bed. Fujimoto proposes conducting series ofrelative gravimetric measurements with a gravity sensor and relativedepth measurements with a depth sensor on survey stations arranged on abenchmark having fixed vertical position relative to the local sea-bedin a survey area over a suspected fault zone, the gravimetricmeasurements being relative to gravimetric measurements and depthmeasurements taken on a reference station on land. Fujimoto proposescorrecting the relative gravimetric measurements for the correspondingrelative depth measurements, producing corrected relative gravityvalues. The corrected gravimetric values are then used for interpretingseabed vertical motions, and not used for comparison between series ofobserved corrected gravimetric values with later series of observedcorrected gravity values and interpretion of a difference of correctedgravimetric values in terms of a change of parameters representingdensity and/or mass displacement in the underground sub-sea reservoir.Although seabed subsidence monitoring over the reservoir zone is onemajor issue of the present invention, the gravity change represented byseabed subsidence is noise with respect to detecting gravity changes dueto mass movements and density change in the reservoir. Thus, in thepresent invention, the gravity measurement due to seabed subsidence (orrise) must, in addition to tidal and drift corrections, be corrected forby corresponding water column pressure changes at the seabed.

Presentation of Relevant Known Art

Gravity monitoring has previously been applied in exploitinghydrothermal energy (Allis, R. G, and Hunt, T. M., Geophysics 51, pp1647-1660, 1986: Analysis of exploitation-induced gravity changes atWairakei geothermal field.; San Andres, R. B, and Pedersen, J. R.,Geothermics 22, pp 395-422, 1993, Monitoring the Bulalo geothermalreservoir, Philippines, using precision gravity data.), and involcanology (Rymer, H. And Brown, G. C., J. Volcanol. Geotherm. Res. 27,pp. 229-254, 1986, Gravity fields and the interpretation of volcanicstructures, geological discrimination and temporal evolution.).Recently, efforts on measuring gravity differences above hydrocarbonfields on land have been reported (Van Gelderen, M., Haagmans, M. R.,and Bilker, M., Geophysical Prospecting 47, pp. 979-993, 1996, Gravitychanges and natural gas extraction in Groningen). For offshore fields,gravity monitoring has been initiated in a case known to us (Hare, J.,Ferguson, J. F., Aiken, C. L. V., and Brady, J. L., 1999, The 4-Dmicrogravity method for waterflood surveillance: A model study for thePrudhoe Bay reservoir, Alaska.), with measurements being performed fromthe surface of the ocean ice, a non-actual situation for mosthydrocarbon fields in question. The relatively small gravity changesexpected due to reservoir parameters as compared to the gravityvariations due to noise and external variations as tides and diurnalgravity variations, requires better accuracy than has been achieved inmarine geophysical surveys to date.

Existing underwater gravity meter systems are based upon lowering thegravity instrument from a ship (LaCoste, 1967; Hildebrand et al., 1990).Other systems make use of manned submersible vessels for takingmeasurements from within the crew compartment of the submersible vessel(Holmes and Johnson, 1993; Cochran et al, 1994, Evans, 1996; and Balluet al., 1998). The problem of relocating the gravity meter observationpoint relative to the seafloor to within approximately 2 centimeters inthe vertical line, is pronounced in the attempts of using a gravitymeter inside a manned submersible vessel. The method is also slow inoperation, and highly expensive, and running a manned submersible vesselmay pose a risk to the crew. Wave action on the vessel is another noiseacting on the gravity meter, and so is the noise due to inadvertentmovement of the crew.

Actual Problems Implied with the Known Art

Gravity measurements taken on a surface ship or inside a submarine inmotion require absolute velocity and course determination in order toperform an Eötvös correction. Shipborne measurements of gravity arenotoriosly noisy due to the ship's accelerations from sea waves andwind, so the measurements must be low-pass filtered over long periods.

General navigation problems makes repeat measurements made by submarineor ROV uncertain with respect to position and elevation. The elevationuncertainty depends on the uncertainty of horisontal position and thelocal inclination of the seabed. The position and elevation problem ofthe known art is remedied by the present invention.

Another problem is represented by the generally unconsolidatedsedimentary seabed surface. The unconsolidated sedimentary surface givesinconsistent subsidence of the gravity measurement package, either beinga bottom gravity meter lowered in a cable from a ship or set out by anROV, or measurements taken from inside a manned submarine resting on theseabed.

Drift of the gravity meter requires frequent reoccupations to thereference station. Thus the long transport time to a land-basedreference station makes frequent returns to a land-based referencestation unfeasible.

Use of a sea-bed reference station in a shaft near the seabed would notsolve the problem with gravimeter drift of the field instrument beingcarried around by an ROV.

Solution to the Problem and Reference to the Claims

The above-mentioned problems are largely reduced by a method for depthmeasurements and monitoring of a seabed subsidence due to compaction ina hydrocarbon reservoar according to the invention defined in theattached set of claims.

The method according to the invention removes the need for making anEötvös correction for vessel speed and vessel course of the gravitymeasurements because the measurements according to the present methodare made stationary at the seabed. The measurement of gravity and depthare done stationary, thus no velocity corrections are needed.

The method according to the invention using measurement stations onpreinstalled benchmarks removes the measurement position uncertainty ofposition reoccupation to within the small area of the top surface of thebenchmark, thus also removing a significant portion of the uncertaintyof elevation reoccupation due to seabed inclination, by the same means.

The method according to the invention using measurement stations onpreinstalled measurement stations on benchmarks reduce the problems with“rapid” subsidence of the measurement vessel sinking in the loosesedimentary seabed due to the softness and unstability of the upperunconsolidated layers of the sediments. Heavy benchmarks which areperforated and made in concrete are preinstalled at the seabed and leftto settle in the sediments for several weeks or months before a firstseries S₁ of gravity and depth measurements. By this, two essentialproblems are solved:

(a) The elevation of the measurement station (with respect to the localconsolidated seabed, not with regard to the earth's gravity centre) isconstant to within millimeters during a series of measurement seriesS₁,S₂, . . . , S_(m) taking place during several months or years.

(b) Vertical (and possibly horizontal) acceleration experienced duringto slow sinking and slow settling of the measurement vessel in the localunconsolidated sediments may explain the gravity phenomenon described inFujimoto et al., see FIG. 5 and the text at p. 910, at the bottom of theleft column.

The measurement stations at preinstalled benchmarks according to thepresent invention prevents such sinking and settling accelerationsduring the measurement at each particular station.

The method according to the invention using a reference station at theseabed in the vicinity of the survey area 8 makes the return time to thereference station on the order of hours. Reoccupying a reference stationonshore, as done in the known art, requires returning the gravity vesselfrom the seabed to the surface and making a gravity measurement eitheron land or at shallow depth.

All transport may incur mechanical stress that may change theinstrument's drift rate.

In the known art, the depth or pressure sensor is arranged on thesubmarine's outer hull surface or other places which may not be exactlyof the same relative depth with respect to the gravity sensor. Accordingto the present invention, the depth or pressure sensor is arranged inthe same elevational position with respect to the gravity sensor, thepressure sensors being arranged on the outside of the gravity sensorwater-tight pressure housing. Thus the relative depth between thegravity sensor and the pressure sensors should be repeatable within farless than 1 cm, depending only on the tilt of the water-tight housingresting on the station on the benchmark.

None of the existing systems are capable of meeting the geophysicalaccuracy, operational speed and economical requirements of the taskpresented by the actual monitoring of a subsea gas reservoir, togetherwith the need to precisely relocating the gravity instrument on theseafloor. On this background, the inventors came up with a new methodand a new instrument according to the invention, solving the problems ofthe disadvantages of the known art.

SHORT DESCRIPTION OF THE DRAWING FIGURES

FIG. 1a illustrates the pressure and gravity sensor package connected toan ROV, further connected to a surface ship.

FIG. 1b illustrates a seafloor geophysical setting, with seafloorsubsidence occuring at long term, and with an underground reservoir tobe monitored.

FIG. 1c illustrates a modeled gravity field change due to a change in areservoir.

FIG. 2 illustrates a setup of instrument stations at benchmarks at theseabed.

FIG. 3 illustrates subsidence of the seafloor and illustrates a methodof monitoring seafloor subsidence, and a corresponding gravity and depthparameter measurement matrix built up during a long-term series ofmeasurements.

FIG. 4 illustrates a benchmark according to the invention, forinstallation on and settling into the seabed sediment surface.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Mechanical System

The preferred embodiment of the gravity and pressure sensor system isshown in FIG. 1a and comprises a sea-floor deployable system 1 having agravity sensor system 3 comprising a gravity sensor 10 in a water-tightpressure housing 34 provided with thermal isolation 35. A double-gimbalframe 36 comprise two orthogonally arranged gimbal frames 36 x and 36 ysupporting the gravity sensor housing 34 free to swing +/−8 degreesabout an x-axis 37 x and +/−9 degrees about a y-axis 37 y. Thus a tiltof +/−12 degrees is allowed about the x-y coordinate diagonal axis. Thisgiven freedom to swing about two orthogonal axes has proved sufficientfor the tilts encountered in a field test illustrated in FIG. 1b showingthe gravity sensor system 3 placed on a benchmark 6 arranged on a localsea-bed station 14 a, . . . ,14 n in a survey area 8, or a referencestation sea-bed 14 r. The preferred embodiment of the benchmark 6 willbe described below. One or more depth sensors 12 are arranged outsidethe pressure housing 34 for a fixed relative elevation in theiroperational position on the station 2,9 with respect to the gravitysensor 10.

The Method According to the Invention; Subsidence

A method is illustrated in FIG. 1b and also in FIG. 3, for monitoringsubsidence of the sea-bed 14 of a survey area 8 caused by compaction ofan underground hydrocarbon reservoir 1. The method of the inventioncomprises the following steps:

Conducting at least two series (S₁, . . . , S_(m)) of depthmeasurements, each series S_(i) comprising at least one time-indexeddepth measurement 13 a,13 b, . . . ,13 n. Depth mesurement 13 a ₁ isconducted at a seabed station 2 a during measurement series S₁;measurement 13 b ₁ is conducted on station 2 b during series S₁, depthmesurement 13 a ₂ is conducted at seabed station 2 a during measurementseries S₂, and so on. A separation in time Δt between at least two ofthe measurement series (S₁, . . . ,S_(i), . . . ,S_(m)), i.e. at leastthe time between the first and the last series S₁ and S_(m) should becharacteristic of a significantly detectable long-term change ofseafloor elevation, possibly due to compaction, to take place in thereservoir 1, if such a change occurs. The tidal measurements should betime-indexed in order to be corrected for tidal depth variations whichwill be described below. In practice, the depth measurements areconducted by measuring the absolute pressure, being proportional to theweight of the unit area water column and the air column above thepressure sensor. A clock (16) may time-index each depth measurement 13or preferably each gravity measurement 11 described later, for tidal,drift and reference corrections of the measurements.

The top of the seabed may be very loose and unconsolidated, being a badbasis for gravity measurements. In this method, it is necessary to takedepth measurements 13 a,13 b, . . . ,13 n relative to the locallyconsolidated seabed, i.e. the seabed which is stable relative to theimmediately underlying geological consolidated ground. To achievestructurally stable survey stations 2 a,2 b, . . . ,2 n, these arearranged on benchmarks 6 a,6 b, . . . ,6 n, the benchmarks preferablybuilt in concrete, arranged at the seabed. Such benchmarks areillustrated in detail in FIG. 5, and are also illustrated at the seabedin FIGS. 1b, 2 and 3. Each depth measurement 13 a,13 b, . . . ,13 n isconducted on such a survey station 2 a,2 b, . . . ,2 n arranged on abenchmark 6 a,6 b, . . . ,6 n. Each benchmark is having fixed verticaland horizontal position relative to the local sea-bed 14 a,14 b, . . .,14 n. Even though the very top of the sedimentary layers may be looseand unconsolidated, each benchmark according to the invention may sinkto settle, and will be consolidated in the sediments before commencementof the first measurement series S₁.

Short-term depth variations comprises the effects of tides, localcurrents and air pressure variations. In order to handle short-termdepth variations during a measurement series S_(i), at least onestationary time-indexed short-time local reference depth measurementseries 19 r may conducted at a short-term local reference station 18 rat such a benchmark 6. Preferably there are several such measurementseries 19 r being conducted during each separate measurement seriesS_(i). The stationary short-time local reference depth measurementseries 19 r are preferably continuous, and are used for correcting eachdepth measurement 13 a,13 b, . . . ,13 n for short-time (e.g. tidal)depth variations occuring during each separate measurement series S_(i).

In order to monitor compaction or seafloor subsidence, the depthmeasurements 13,19 r are conducted relative to at least one depthmeasurement 13 r at a reference station 9 arranged on the seabed 14 routside the survey area 8. This reference depth measurement is taken atleast once during each measurement series S₁. The reference station 9must be arranged at a distance from the reservoir 8 far enough to beingunaffected by long-term effects taking place due to seabed subsidence orcompaction in the reservoir 1 during the series of measurements S₁, . .. ,S_(m), but near enough both with respect to depth and distance to bereached within reasonable time during each series of measurements, andsituated at a depth comparable to the depth of the survey area to reducethe depth span and most of all to reduce drift of the gravity meter andto avoid uncertainty with respect to ground density for gravityelevation corrections.

The seafloor subsidence may be monitored as a difference of relativedepth values Δd or 15 ₂, . . . ,15 _(m) having occured during the timeΔt and interpreted in terms of compaction in the reservoir 1.

In a preferred embodiment of the invention, the stationary short-timelocal reference depth measurement 19 r series is conducted by at leastone separate depth meter 17 r arranged at at least one survey station18, which may be at least one of the survey stations 2 in the surveyarea 8.

The instrument 17 r measuring the reference depth measurement series 19r is preferably a continuously registering and logging depth meter. Themeasurements 19 r may be used for tidal corrections. Predicted orcalculated tidal corrections without local measurements may not besufficiently accurate for use in the method of the invention. In apreferred embodiment of the invention, preferably three separate depthmeters 17 r ₁, 17 r ₂ and 17 r ₃ are spread at separate short-termreference stations 18 r ₁ , 18 r ₂ and 18 r ₃ distributed over thesurvey area 8 to monitor the tidally and geographically varying seriesof depth measurements 19 r. The three separate continuous and timeindexed depth measurement series 19 r ₁, 19 r ₂, 19 r ₃ may be used witha tidal model for interpolating the local tidal depth for correctingeach time-indexed depth measurement 13 at each station 2.

Preferably, at least one stationary short-time local reference depthmeasurement 19 r series is conducted at a short-term local referencestation 18 r being identical to or co-located with the reference station9 arranged on the seabed 14 r outside the survey area 8.

Gravity Combined with Depth Measurements

Having the depth measurements available for controlling or monitoringthe gravity effect of the sea-water masses and of the seafloorsubsidence, monitoring parameters representing density and/or massdistribution in an underground sub-sea petroleum reservoir 1 is possibleby means of gravimetric measurements 11 with a mobile gravity sensor 10as described above, for use with an ROV 5 on the sea-bed 14 of a surveyarea 8 covering the petroleum reservoir 1. The novel features of themethod according to the invention are the following steps comprisinggravity measurements:

Including, during the same measurements series S₁, . . .,S_(m), relativegravmetric measurements 11 a,11 b, . . . ,11 n, preferably acquiredsimultaneously with the depth measurements 13 a,13 b, . . . ,13 n.

Each gravity measurement 11 a,11 b, . . . ,11 n is conducted onessentially the same survey station 2 a,2 b, . . . ,2 n arranged on thebenchmarks 6 a,6 b, . . . ,6 n as for the depth measurements 13 a,13 b,. . . ,13 n.

The gravity measurement 11 is conducted relative to at least one pair ofreference gravity measurement 11 r and a reference depth measurement 13r or 19 r at a reference station 9 or 18.

The relative gravimetric measurements 11 are corrected for thecorresponding long-term and short-term relative depth measurements 13 or19 r and provides depth corrected relative gravity values 21 a,21 b, . .. 21 n.

The relative gravity values 21 are corrected for the effect of seabedsubsidence 15 ₂, . . . ,15 _(m) as calculated on the basis of therelative depth measurements 13 a,13 b, . . . ,13 n during the long-termtime Δt.

The difference of depth corrected relative gravity values Δg or Δ21₂,Δ21 ₃, . . . ,Δ21 _(m) having occured during the long-term time Δt maybe interpreted in terms of parameters representing a mass density changeand/or a mass displacement in the reservoir 1.

The time interval Δt between at least the first and the last measurementseries may be on the order of months, one year or longer time. In orderto monitor gravity effects, the separation in time Δt between themeasurement series S₁, . . . ,S_(m) must be sufficient for orcharacteristic of a significantly detectable change of gravity Δg totake place in the reservoir, the change of gravity being due to a changeof mass density. Such a long time span would normally be the same as fora significantly measurable seafloor subsidence to take place, but is nolimiting condition for the gravity method to work. A significantlydetectable change of gravity Δg may be imagined to take place due to adensity change or fluid movement occuring in the reservoir, without anysignificantly measurable seafloor subsidence or even seafloor rise dueto other geological processes.

Each gravity measurement 11 a,11 b, . . . ,11 n is conducted with thegravity and depth sensors 10,12 placed on a survey station 2 a,2 b, . .. ,2 n. The survey stations are arranged each on their benchmark 6 a,6b, . . . ,6 n having fixed vertical and horizontal position relative tothe local sea-bed 14 a,14 b, . . . ,14 n. The benchmarks are arranged inknown and marked positions on the seabed in sufficient time before thegravity survey series S₁, . . . , S_(m) for the benchmarks to settlefirmly and stably at the seabed. One preferred embodiment of a benchmark6 is shown in FIG. 4. The benchmark 6 has a skirt below the outerperiphery, and a somewhat elevated top surface constituting the surveystation 2. The baseplate of the benchmark 6 is perforated in order forwater and loose sediment to be pressed through the benchmark during thesettling in the sediments. The benchmark is made in concrete and isarranged to sink into the sediment leaving at least the elevated topsurface above the sediments when settled.

The gravity and depth measurement 11,13 are made relative to at leastone gravity and depth measurement 11 r,13 r at a reference station 9arranged on the seabed 14 r outside the survey area 8. This isillustrated in the simplified FIG. 2. The reference station 9 is farenough to be unaffected by gravity effects taking place due to masschanges in the reservoir 1, e.g. due to an elevating gas/water contactGWC or an elevating oil/water contact OWC. A modelled example of therelative gravity effect of a 10 meters uplift of the gas/water contactin the Troll reservoir is shown in FIG. 1c. The relative gravity valuesare shown in microGals. One can see that the gravity image is a smoothedimage of the reservoir contour. The station grid has a spacing of about4 km, with the stations 2 indicated by crosses.

The relative gravimetric measurements 11 must be corrected for thecorresponding relative depth measurements 13 producing depth correctedrelative gravity values 21 a,21 b, . . . ,21 n.

In order to discriminate a gravity effect due to subsidence from gravityeffect due to a mass density change and/or a mass displacement in thereservoir 1, the relative gravity values 21 should be corrected back toa datum plane, for the effect of seabed subsidence 15 ₂, . . . ,15 m ascalculated on the basis of the relative depth measurements 13 a,13 b, .. . ,13 n during the time Δt. This subsidence measurement and correctionis illustrated in FIG. 3. The datum plane illustrated is a plane throughthe reference station 9 at the seabed 14 r outside the survey area 8,but other datum planes closer to the stations 2 may serve the correctionpurpose better.

After correcting the gravity values 11 for subsidence measurement, onecan interpret a difference of depth corrected relative gravity values Δgor Δ21 ₂,Δ21 ₃, . . . 66 21 _(m) having occured during the time Δt interms of parameters representing a mass density change and/or a massdisplacement in the reservoir 1. This displacement is illustrated inFIG. 1b by the gas/water contact from the first series S₁ beingindicated by GWCt₁ and the gas/water contact from the second seriesS_(m) being indicated by GWCt_(m). This rise of GWC may be calculatediteratively until the modelled gravity change fits the measured gravitychange, or may be inversely modeled directly from the data. The densitychange at the gas-water contact level may also be inferred fromtwo-dimensional spatial deconvolution of the seafloor gravity data.Several methods for modeling of gravity data are known from theliterature, and such methods are not described in detail here.

During each and every measurement series S₁, S₂, . . . , S_(m) thegravity sensor 10 and the depth sensor 12 are arranged with a fixedrelative elevation in their operational position on the station 2,9.This is made practical by arranging the depth sensors 12 on the outsidethe pressure housing of the gravity meter as illustrated in FIG. 1a.

The gravity sensor 10 and the depth sensor 12 are carried by means of anROV 5 from one station 2, 9 to another station 2, 9 between one pair ofa relative gravimetric and depth measurements 11,13 and the next pair ofa relative gravimetric and depth measurement 11,13 in one measurementseries S. By using a cable-powered ROV 5 there is no limit to the sizeof the survey area.

According to a preferred embodiment of the invention, in order not todisturb the sensors, particularly the gravity sensor, the ROV 5separates from and leaves the gravity sensor 10 (and the depth sensor12) before the commencement of each relative gravimetric measurement 11in order not to affect the gravimetric measurment 11.

According to the preferred embodiment, the relative gravimetricmeasurement 11 from the gravity sensor 10 and the depth measurement 13from the depth sensor 12 are transferred from the unit 4 to the ROV 5via a “ROVDOG” umbilical cable 53.

According to the preferred embodiment, the relative gravimetric 11 anddepth measurements 13 are transferred via an ROV umbilical cable 51 to asurface vessel 7. By this arrangement the crew can monitor online andcontrol the sampling of gravity and depth data.

The gravimetric measurements 11 are time-indexed and corrected for thegravity sensor's 10 drift with respect to time by other time-indexedgravimetric measurements 11, 11 r taken before and/or later with thesame gravity sensor 10 at a survey station 2 or reference station 9during the same actual period of time t_(i), giving drift-correctedgravimetric measurements 11 _(t) for further processing to producecorrected gravity values 21. The advantage by the present method is thatthe reference station may be reoccupied often during the survey. In thisway the drift rate of the gravity meter can be monitored.

According to a preferred embodiment of the invention, the depthmeasurements 13 arise from pressure measurements 23 converted accordingto the actual water density, the depth sensors 12 actually beingpressure sensors. Optionally, a measured water density distributiondepth profile is taken into account.

A tidal water correction can be made for the corresponding relativedepth measurements 13 on each series S_(i) of the gravimetricmeasurements 11 in order to obtain corrected relative gravity values 21.The tidal water correction can be made theoretically on the basis oftidal modelling, or based upon gravity measurements during reoccupationof reference stations 9 or gravity stations 2.

Further Description of the Mechanical Structure

Each gimbal frame 36 x, 36 y is driven by an actuator 38 x, 38 y. Theactuator 38 x may rotate the gimbal frame 36 x about the x-axis 37 x,and the actuator 8 y is arranged to rotate the gimbal frame 36 y aboutthe y-axis 37 y. Each actuator 38 x, 38 y comprises a DC motor-drivenlead screw 80 x, 80 y whose position is monitored by a linearpotentiometer 82 x, 82 y. Each linear potentiometer 82 x, 82 y gives areadout 83 x, 83 y which is transmitted via a signal conductor 84 x, 84y to a control device 88 arranged to control the actuators 8 x, 8 y.

Electrical System

Control of the system is overseen by a microcontroller 30 (Z-World modelBL1700) being a single-board computer comprising four serial ports, 1012-bits A/D converters, and 64 digital I/O lines. The A/D channels arearranged to monitor signals comprising the following:

a) A temperature signal 61 from a temperature sensor 60;

b) A sensor coarse tilt signal 65 from a coarse tilt sensor 64;

c) A sensor fine tilt signal 69 from a fine tilt sensor 68;

d) An ambient temperature signal 71 from a temperature sensor 60;

e) Gimbal orientation sensor signals 73 x, 73 y (different from thesignal 83 x, 83 y mentioned above) from gimbal orientation sensors 72 x,72 y giving the orientation of gimbals 80 x, 80 y;

f) A motor current size signal 75 from a motor current sensor 74;

g) a leak signal 91 from a leak sensor 90.

Component and function list:  1 sub-sea reservoir.  2 survey station(2a, . . . , 2n) on benchmark 6 on seabed 14.  3 gravity sensor systemcomprising gravity sensor 10.  4 ROVDOG comprising 3 and depth(pressure) sensors 12.  5 ROV.  6 benchmark on seabed 14.  7 surface (orsubmarine) vessel.  8 survey area.  9 Fixed reference station withrespect to the seabed 14r. 10 gravity sensor. 11 relative gravimetricmeasurements (11a, . . . , 11n). 12 depth sensor. 13 relative depthmeasurements (13a, . . . , 13n. 14 local survey area sea- bed (14a, . .. , 14n). reference station sea- bed (14r). 15 relative depth values(Δd) or (15₂, . . . ,15_(m)) 16 clock (16) 17 separate depth meter (17r)18 short-term local reference station. 19 short-time local referencedepth measurement series. 20 21 corrected gravity values (21a, 21b, . .. , 21n). 22 quartz pressure gauges 23 pressure measurements (23a, 23b,. . . , 23n). 24 25 26 27 28 29 30 microcontroller. 31 32 33 differenceof relative depth values (Δd) or (Δ33₂, Δ33₃, . . . , Δ33_(m)). 34water-tight housing. 35 thermal isolation. 36x, y double-gimbal framesorthogonally arranged. 37x, y x-axis, y-axis 37x, 37y. 38 actuators 38x,38y for gimbal frames 36x, 36y. 39 40 41 42 43 44 45 46 47 48 49 50 51ROV umbilical cable 52 53 ROVDOG umbilical cable 54 55 56 57 58 59 60temperature sensor. 61 temperature signal from 60. 62 63 64 coarse tiltsensor. 65 coarse tilt signal from 64. 66 67 68 fine tilt sensor. 69fine tilt signal from 68. 70 ambient temperature sensor. 71 ambienttemperature signal from 70. 72 73 74 75 76 77 78 79 80 motor-driven leadscrew 80x, 80y. 81 82 linear potentiometer 82x, 82y monitoring 80x, 80y.83 readout 83x, 83y from 82. 84 signal conductor 84x, 84y. 85 86 87 88control device. 89 90 leak sensor. 91 leak signal from 90.

What is claimed is:
 1. A method for monitoring possible subsidence of aseabed (14) of a survey area (8) caused by compaction of an undergroundhydrocarbon reservoir (1), comprising: conducting at least twomeasurement series (S₁, . . . ,S_(i), . . . ,S_(m)) each comprising atleast one time-indexed depth measurement (13 a,13 b, . . . ,13 n), witha separation in time Δt between the at least two measurement series onthe order of months or years; conducting each depth measurement (13 a,13b, . . . ,13 n) on a survey station (2 a,2 b, . . . ,2 n) arranged on abenchmark (6 a,6 b, . . . ,6 n) having fixed vertical and horizontalposition relative to the local sea-bed (14 a,14 b, . . . ,14 n); withineach measurement series (S_(i)), conducting at least one stationarytime-indexed short-time local reference depth measurement series (19 r)on at least one short-term local reference station (18 r) on at leastone benchmark (6), for correcting each depth measurement (13 a,13 b, . .. ,13 n) for short-time depth variations; conducting the depthmeasurements (13,19 r) relative to at least one depth measurement (13 r)at a reference station (9) arranged on the seabed (14 r) outside thesurvey area (8) at least once during each measurement series (S_(i)),the reference station (9) essentially being unaffected by long-termeffects taking place due to compaction in the reservoir (1) during theseries of measurements (S₁, . . . ,S_(i), . . . , S_(m)).
 2. Methodaccording to claim 1, comprising interpreting a difference of relativedepth values (Δd) or (15 ₂, . . . , 15 m) having occured during the time(Δt) in terms of compaction in the reservoir (1).
 3. Method according toclaim 1, the time between at least the first and the latest measurementseries (S₁, . . . ,S_(m)) being characteristic of a significantlydetectable change of seafloor elevation due to compaction to take placein the reservoir (1).
 4. Method according to claim 1, the depthmeasurements (13 a, 13 b, . . . , 13 n) being deducted from pressuremeasurements (23 a, 23 b, . . . , 23 n).
 5. Method according to claim 1,the stationary short-time local reference depth measurement series (19r) being continuous.
 6. Method according to claim 1, the referencestation (9) essentially being close to the survey area (8) and situatedat a depth comparable to the depth of the survey area.
 7. Methodaccording to claim 1, comprising the stationary short-time localreference depth measurement (19 r) series being conducted by at leastone separate depth meter (17 r).
 8. Method according to claim 1,comprising at least one stationary short-time local reference depthmeasurement (19 r) series being conducted at a short-term localreference station (18 r) being identical to or co-located with thereference station (9) arranged on the seabed (14 r) outside the surveyarea (8).
 9. Method according to claim 1, comprising arranging three ormore separate depth meters (17 r ₁,17 r ₂,17 r ₃) at separate short-termreference stations (18 r ₁,18 r ₂,18 r ₃) distributed geographicallyover the survey area (8) to monitor the tidally varying series of depthmeasurements (19 r), using the separate continuous and time indexeddepth measurement series (19 r ₁,19 r ₂,19 r ₃) with a tidal model forinterpolating the local tidal depth for correcting each time-indexeddepth measurement (13) at each station (2).
 10. A method according toclaim 1, further comprising: including, in at least two of themeasurements series (S₁, . . . ,S_(i), . . . S_(m)), relativegravimetric (11 a,11 b, . . . ,11 n) measurements simultaneously withthe depth measurements (13 a,13 b, . . . ,13 n); conducting each gravitymeasurement (11 a,11 b, . . . ,11 n) on the survey stations (2 a,2 b, .. . ,2 n) arranged on the benchmarks (6 a,6 b, . . . ,6 n); conductingthe gravity measurement (11) relative to at least one reference gravitymeasurement (11 r) at the reference station (9); correcting the relativegravimetric measurements (11) for the corresponding long-term andshort-term relative depth measurements (13,19 r) producing depthcorrected relative gravity values (21 a,21 b, . . . ,21 n); correctingthe relative gravity values (21) for the effect of seabed subsidence (15₂, . . . ,15 _(m)) as calculated on the basis of the relative depthmeasurements (13 a,13 b, . . . ,13 n) during the long-term time Δt; andinterpreting a difference of depth corrected relative gravity values(Δg) or (Δ21 ₂, Δ21 ₃, . . . ,Δ2l_(m)) having occurred during thelong-term time (Δt) in terms of parameters representing a mass densitychange and/or a mass displacement in the reservoir (1).
 11. A methodaccording to claim 5, in which during each measurement series (S₁, S₂, .. . , S_(m)) the gravity sensor (10) and the depth sensor (12) arearranged with a fixed relative elevation in their operational positionon the station (2, 9, 18).
 12. A method according to claim 5, in whichthe gravity sensor (10) and the depth sensor (12) are carried by meansof an ROV (5) from one station (2, 9) to another station (2, 9) betweenone pair of a relative gravimetric and depth measurements (11,13) andthe next pair of a relative gravimetric and depth measurement (11,13) inone measurement series (S).
 13. A method according to claim 12, in whichthe ROV (5) is separate from the gravity sensor (10) during eachrelative gravimetric measurement (11) in order not to affect thegravimetric measurment (11).
 14. A method according to claim 12, inwhich the relative gravimetric measurement (11) from the gravity sensor(10) and the depth measurement (13) from the depth sensor (12) aretransferred to the ROV (5).
 15. A method according to claim 14, in whichthe relative gravimetric (11) and depth measurements (13) aretransferred via an ROV umbilical cable (53) to a surface vessel (7). 16.A method according to claim 10, in which the gravimetric measurements(11) are time-indexed and corrected for the gravity sensor's (10) driftwith respect to time by other time-indexed gravimetric measurements (11,11 r) taken before and/or later with the same gravity sensor (10) at asurvey station (2) or reference station (9,18) during the same actualperiod of time (t_(i)), giving drift-corrected gravimetric measurements(11 _(t)) for further processing to produce corrected gravity values(21).
 17. A method according to claim 4, wherein the depth measurements(13) arise from pressure measurements (23) converted according to theactual water density and optionally to the measured water densitydistribution depth profile.
 18. Device for monitoring possiblesubsidence of a seabed (14) of a survey area (8), and gravity changes inan underlying petroleum reservoir, comprising: a depth sensor (12),adapted for being carried by an ROV (5), to be placed on survey stations(2) on benchmarks (6) at the seabed (14) for conducting depthmeasurements (13); a water-tight pressure housing (34) with a gravitysensor (10) for making relative gravity measurements (11); in which thegravity sensor (10) and the depth sensor (12) are arranged in a mutuallyfixed elevation in their operational position, and adapted for beingcarried by means of an ROV (5) from one seabed station (2, 9) to anotherseabed station (2, 9) between one pair of a relative gravimetric anddepth measurements (11,13) and the next pair of a relative gravimetricand depth measurements (11,13).
 19. Device according to claim 18, thedepth sensor (12) comprising quartz pressure gauges (22).
 20. Deviceaccording to claim 18, the number of pressure gauges (22) being three.21. Device according to claim 18, comprising a clock (16) fortime-indexing the depth measurements (13).
 22. Device according to claim21, comprising double gimbal frames (36 x,36 y) orthogonally arrangedfor adjusting the verticality of the gravity sensor (10) around two axes(37 x, 37 y).
 23. Device according to claim 22, comprising actuators (38x,38 y) for turning the gravity sensor (10) in the gimbal frames (36x,36 y).
 24. Device according to claim 21, comprising coarse and finetilt sensors (64,68) for giving a coarse and fine tilt signal (65,69)for the deviation from verticality for the gravity sensor (10). 25.Device according to claim 21, comprising a signal cable (53) between thedepth sensor (12) and the gravity sensor (10), and the ROV (5), arrangedfor transmitting the depth measurement (13) and the relative gravitymeasurement (11) from the sensors (10,12) to the ROV.
 26. Deviceaccording to claim 25, the signal cable (53) also conducting the tiltsignals (65,69) from the tilt sensors (64,68) of the gravity sensors tothe ROV (5).
 27. Device according to claim 18, at least the gravitysensor (10) adapted for being released from the ROV (5) during eachrelative gravimetric measurement (11) in order not to be disturbed bythe ROV (5).
 28. A device according to claim 18, in which the relativegravimetric measurement (11) from the gravity sensor (10) and the depthmeasurement (13) from the depth sensor (12) are transferred to the ROV(5) by an umbilical cable (53).
 29. A device according to claim 14, inwhich the relative gravimetric (11) and depth measurements (13) aretransferred via an ROV umbilical cable (51) to a surface vessel (7).